BACKGROUND OF THE INVENTION
I. Field of the Invention
[0001] This invention relates to a motor-driven power steering system for a vehicle adapted
to assist the operator-induced steering operation by means of the rotative force of
a motor.
2. Description of the Prior Art
[0002] In the past, a conventional power steering system of this type, as schematically
illustrated in Fig. has hitherto been known in which a steering wheel I is operatively
connected through a steering shaft 2 having an upper portion 2a and a lower portion
2b and a first rack and pinion gear Ti including a first pinion 5 and a first rack
tooth portion 6a with a steering rack 6 which is connected at its opposite ends with
a pair of steerable road wheels (not shown) through a pair of tie rods 8a, 8b so that
when the steering wheel I is turned by an operator, the steerable road wheels (not
shown) are appropriately steered in accordance with the steering motion of the steering
wheel I imparted bythe operator. On the other hand, the steering rack 6 is operatively
connected with a motor 13 through a second rack and pinion gear T
2 including a second rack tooth portion 6b and a second pinion 17, a speed-reduction
gear R, and a switching clutch 16 so that the driving force of the motor 13 is transmitted
through the speed-reduction gear R, the switching clutch I6 and the second rack and
pinion gear T
2 to the rack 6 so as to assist the steering operation of the steering wheel I imparted
by the operator. The motor 13 is electrically connected with a battery II through
a control unit 9 and a key or ignition switch 12 so that it is energized by the battery
It under the control of the control unit 9. The control unit 9 is input with control
signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately
control the operations of the motor 13 and the switching clutch 17 on the basis of
the steering torque and the vehicle speed measured. However, the power steering system
as constructed above has the following problems. Specifically, when the travelling
speed of the vehicle increases above or decreases below a prescribed level, the control
unit 9 operates to deenergize or energize the switching clutch 17 to interrupt or
enable the transmission of the assisting force from the motor 13 to the rack 6 so
as to switch the steering system into a manual steering mode without any power assist
or into a power steering mode with power assist. In this case, if the speed sensor
3 has failed or an electrical line or wiring connecting the speed sensor 3 to the
control unit 9 is broken for some reason, no output signal is fed to the control unit
9 so that the motor 13 continues to operate with the clutch 16 connected. As a result,
the steering system is continuously held in a power steering mode even when the vehicle
speed exceeds the prescribed speed level. Such a situation is unfavorable from the
standpoint of safety in driving of the vehicle.
SUMMARY OF THE INVENTION
[0003] In view of the above, the present invention is intended to obviate the above-described
problem of the prior art, and has for its object the provision of a motor-driven power
steering system for a vehicle which can ensure safety in high-speed driving of the
vehicle even when the speed sensor has failed or the wiring connecting between the
speed sensor and the control unit is broken.
[0004] In order to achieve the above object, according to the present invention, there is
provided a motor-driven power steering system for a vehicle having a steering wheel
operatively connected with steerable road wheels so that the steering wheel is turned
by an operator to appropriately steer the steerable road wheels, the motor-driven
power steering system comprising:
a torque sensor for detecting steering force imparted to the steering wheel by an
operator to generate an output signal representative of the detected steering force;
a vehicle speed sensor for detecting the vehicle speed to generate an output signal
representative of the detected vehicle speed;
a motor adapted to be energized by a source of electric power for power assisting
the steering motion caused by the operator through the steering wheel;
a clutch means adapted to be switched off for interrupting the transmission of the
power-assisting force from the motor toward the steerable road wheels when the vehicle
speed increases above a prescribed level, and switched on for establishing the transmission
of the power-steering force when the vehicle speed decreases below the prescribed
level; and
a control unit connected through wirings to the torque sensor and the vehicle-speed
sensor for controlling the operations of the motor and the clutch means based on the
output signals thereof in a manner such that the power-assisting force transmitted
from the motor to the steerable road wheels is controlled in accordance with the travelling
speed of the vehicle and the steering torque imparted to the steering wheel by the
operator under the normal operating condition of the vehicle speed sensor, but reduced
to zero if the vehicle speed sensor has failed and/or the wiring connecting between
the vehicle speed sensor and the control unit is broken.
[0005] The current supplied from-the source of electric power to the motor is reduced to
zero by the control unit in case of failure in the vehicle speed sensor and/or breaks
in the wiring connecting between the vehicle speed sensor and the control unit. Also,
the clutch means is switched off by the control unit in case of failure in the vehicle
speed sensor and/or breaks in the wiring connecting between the vehicle speed sensor
and the control unit.
[0006] In one embodiment, a vehicle height sensor is provided for detecting the height of
the vehicle from the ground and generate an output signal representative of the detected
vehicle height. In this case, the control unit comprises:
a steering torque measuring means adapted to receive the output signal from the torque
sensor for measuring the operator-induced steering torque;
a vehicle speed measuring means adapted to receive the output .signal from the vehicle-speed
sensor for measuring the vehicle speed;
a vehicle height measuring means adapted to receive the output signal from the vehicle
height sensor and generate an output signal having a variation frequency and a variation
amplitude which correspond to the measured vehicle height;
a vehicle travel judging means adapted to receive the output signal of the vehicle
height measuring means and judge that the vehicle is travelling when the variation
frequency and the variation amplitude of the output signal of the vehicle height measuring
means are continuously greater than respective prescribed reference values for a time
duration exceeding a prescribed period;
a speed-sensor failure determining means adapted to receive the output signals of
the vehicle speed measuring means and the vehicle travel judging means and determine
that there is failure in the vehicle speed sensor and/or breaks in the wiring connecting
between the vehicle speed sensor and the control unit when the output of the vehicle
speed measuring means is zero and when the vehicle travel judging means judges that
the vehicle is travelling;
a motor-current determining means adapted to determine an appropriate level of current
supplied to the motor in a manner such that the motor current is determined on the
basis of the output signals of the steering torque measuring means and the vehicle
speed measuring means in the normal operating condition of the vehicle speed sensor,
but made zero when the speed-sensor failure determining means determines that there
is failure in the vehicle speed sensor and/or breaks in the wiring connecting between
the vehicle speed sensor and the control unit;
a motor-current controlling means for controlling the current supplied to the motor
in accordance with the output from the motor-current determining means; and
a clutch controlling means for controlling the operation of the clutch means in a
manner such that the clutch means is switched off when the measured vehicle speed
is above the prescribed level or when the speed-sensor failure determining means determines
that there is failure in the vehicle speed sensor and/or breaks in the wiring connecting
between the vehicle speed sensor and the control unit, and switched on when the measured
vehicle speed is below the prescribed level.
[0007] In another embodiment, a throttle opening sensor is provided for detecting the opening
degree of a throttle valve disposed in an intake passage of the engine so as to generate
an output signal representative of the detected throttle opening degree. In this case,
the control unit comprises:
a steering torque measuring means adapted to receive the output signal from the torque
sensor for measuring the operator-induced steering torque;
a vehicle speed measuring means adapted to receive the output signal from the vehicle
speed sensor for measuring the vehicle speed;
a throttle opening measuring means adapted to receive the output signal from the throttle
opening sensor for measuring the opening degree of the throttle valve;
a speed-sensor failure determining means adapted to receive the output signals of
the vehicle speed measuring means and the throttle opening measuring means and determine
that there is failure in the vehicle speed sensor and/or breaks in the wiring connecting
between the vehicle speed sensor and the control unit when the opening degree of the
throttle valve measured by the throttle opening measuring means is greater than a
prescribed reference value and when zero output of the vehicle speed measuring means
continues for a time duration exceeding a prescribed period;
a motor-current determining means adapted to determine an appropriate level of current
supplied to the motor in a manner such that the motor current is determined on the
basis of the output signals of the steering torque measuring means and the vehicle
speed measuring means in the normal operating condition of the vehicle speed sensor,
but made zero when the speed-sensor failure determining means determines that there
is failure in the vehicle speed sensor and/or breaks in the wiring connecting between
the vehicle speed sensor and the control unit;
a motor-current controlling means for controlling the current supplied to the motor
in accordance with the output from the motor-current determining means; and
a clutch controlling means for controlling the operation of the clutch means in a
manner such that the clutch means is switched off when the measured vehicle speed
is above the prescribed level or when the speed-sensor failure determining means determines
that there is failure in the vehicle speed sensor and/or breaks in the wiring connecting
between the vehicle speed sensor and the control unit, and switched on when the measured
vehicle speed is below the prescribed level.
[0008] The above and other objects, features and advantages of the present invention will
become apparent from the following detailed description of a few presently preferred
embodiments of the invention when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION-OF THE DRAWINGS
[0009]
Fig. I is a schematic view illustrating the general arrangement of a motor-driven
power steering system for a vehicle having a conventional control unit;
Figs. 2 through 5 relate to a motor-driven power steering system in accordance with
one embodiment of the present invention, in which:
Fig. 2 is a block diagram showing a control unit and its related parts;
Fig. 3 is a characteristic view showing a relationship between the motor current and
the steering torque;
Fig. 4 is a characteristic view showing a vehicle-speed/motor-current relationship
and a vehicle-speed/clutch-voltage relationship; and
Fig. 5 is a flow chart showing the control process of the power steering system illustrated
in Fig. 2.
Figs. 6 and 7 relate to a motor-driven power steering system in accordance with another
embodiment of the present invention, in which:
Fig. 6 is a block diagram showing a control unit and its related parts; and
Fig. 7 relates to a flow chart showing the control process of the power steering system
illustrated in Fig. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] The present invention will now be described in detail with reference to a few presently
preferred embodiments thereof as illustrated in the accompanying drawings. In the
following, the same parts or elements of the embodiments will be identified by the
same reference numerals and characters as employed in Fig. I.
[0011] Referring to the drawings and first to Fig. 2, there is shown a motor-driven power
steering system having a control unit 109 constructed in accordance with one embodiment
of the present invention. As diagrammatically illustrated in Fig. 2, the control unit
109 is adapted to receive the output signals from a torque sensor 3, a vehicle speed
sensor 10 and a vehicle height sensor 18 for controlling the operations of a motor
13 in the form of a DC motor and a clutch means 16 in the form of an electromagnetic
switching clutch in a manner such that the power-assisting force transmitted from
the motor 13 to the steerable road wheels (not shown) is controlled in accordance
with the travelling speed of the vehicle and the steering torque imparted to the steering
wheel I by the operator under the normal operating condition of the vehicle speed
sensor 10, but reduced to zero if the vehicle speed sensor 10 has failed and/or an
electric line or wiring connecting between the vehicle speed sensor 10 and the control
unit 109 is cut or broken.
[0012] Specifically, as shown in Fig. 2, the control unit 109 comprises: a steering torque
measuring means 109a adapted to receive the output signal of the torque sensor 3 for
measuring the operator-induced steering torque; a vehicle speed measuring means 109b
adapted to receive the output signal of the vehicle speed sensor 10 for measuring
the travelling speed of the vehicle; a vehicle height measuring means 109c adapted
to receive the output signal from the vehicle height sensor 18 and generate an output
signal having a variation frequency and a variation amplitude which correspond to
the measured vehicle height; a vehicle travel judging means 109d adapted to receive
the output signal of the vehicle height measuring means 109c and judge that the vehicle
is travelling when the variation frequency and the variation amplitude of the output
signal of the vehicle height measuring means 109c are greater than respective prescribed
reference values for a time duration exceeding a preset period; a speed-sensor failure
determining means 109e adpated to receive the output signals of the vehicle speed
measuring means 109b and the vehicle travel judging means 109d and determine that
there is failure in the vehicle speed sensor 10 and/or breaks in the wiring connecting
between the vehicle speed sensor 10 and the control unit 109 when the output of the
vehicle speed measuring means 109b is zero and when the vehicle travel judging means
109d judges that the vehicle is travelling; a motor-current determining means 109f
adapted to determine an appropriate level of current supplied to the motor 13 in a
manner such that the motor current is determined on the basis of the output signals
of the steering torque measuring means 109a and the vehicle speed measuring means
109b in the normal operating condition of the vehicle speed sensor 10, but made zero
when the speed-sensor failure determining means 109e determines that there is failure
in the vehicle speed sensor 10 and/or breaks in the wiring connecting between the
vehicle speed sensor 10 and the control unit 109; a motor-current controlling means
109g for controlling the current supplied to the electromagnetic switching clutch
16 in accordance with the output from the motor-current determining means 109f; and
a clutch controlling means 109h for controlling the operation of the electromagnetic
switching clutch 16 in a manner such that the clutch 16 is switched off when the measured
vehicle speed is above a prescribed level or when the speed-sensor failure determining
means 109e determines that there is failure in the vehicle speed sensor 10 and/or
breaks in the wiring connecting between the vehicle speed sensor 10 and the control
unit 109, and switched on when the measured vehicle speed is below the prescribed
level.
[0013] Now, the operation of the control unit 109 will be described in detail with reference
to Figs. 3 through 5. First, the case in which a vehicle is stationary or standing
still will be considered. In this case, when a key or ignition switch (not shown)
is first turned on to start the engine, the electromagnetic switching clutch 16 is
automatically actuated to place the second pinion 17 into mechanical coupling with
the worm wheel 15. In this state, when the steering wheel I is turned by an operator,
the control unit 109 acts to control the operation of the motor 13 in the form of
a DC motor in the manner as illustrated in Fig. 3. More specifically, when steering
torque increases in the righthand direction to point a in Fig. 3, the motor 13 is
turned on and supplied with an offset current of a certain low level top (for example,
about 2 to 10 amperes) in order to alleviate the inertia influence from the motor
13 and the mechanical portions. As the steering torque further increases, the current
supplied to the motor 13 begins to increase, at point b, linearly in direct proportion
to the intensity of steering torque, and reaches 100 % current at point c. On the
other hand, as the steering torque decreases, current flowing through the motor 13
begins to decrease at point and becomes to be the offset levelloF at point b. When
the steering torque further decreases and reaches point a, the current supply to the
motor 13 is reduced to zero and the motor 13 is stopped. Similar to this, the motor
13 is controlled in the same manner when the steering torque increases or decreases
in the lefthand direction. In this connection, it is to be noted that the power-assisting
torque generated by the motor 13 is in direct proportion to the current supplied thereto.
[0014] Accordingly, as pictured in Fig. 3, as the steering torque increases, the motor 13
is energized or turned on at point a and supplied with the offset current I
QF. As the steering torque further increases, the current supplied to the motor 13 begins
to gradually increase at point b so that the output torque of the motor 13 to be transmitted
to the worm 14 accordingly increases gradually. As a result, the assisting torque
having the intensity corresponding to the operator's steering effort applied to the
steering wheel I is transmitted to the second rack tooth portion 6b on the steering
rack 6 through the intermediary of the worm wheel 15, the electromagnetic switching
clutch 16 and the second pinion 17, thereby lightening the steering operation for
the operator.
[0015] Now, the case in which the vehicle is travelling will be considered. In this case,
as seen from Fig. 4, the control unit 109 controls the operations of the motor 13
and the electromagnetic switching clutch 16 in the following manner. The current supplied
to the motor 13 is reduced to an offset current level I
OF (about 2 - -10 amperes) at point d in Fig. 4 at which the vehicle speed increases
to a prescribed second reference speed V
2, held constant at the offset current level top until the vehicle speed further increases
to a prescribed first reference speed level Vi at point e in Fig. 4, and reduced to
zero at point e. Also, the voltage imposed on the clutch 16 is held constant at 100%
level until the vehicle speed increases to the first reference speed level Vi at point
e, and made zero at the first reference speed level Vi so that the clutch 16 is switched
off to release the mechanical connection between the worm wheel 15 and the second
pinion 17, thereby making the steering operation for the operator heavier without
any power assist.
[0016] With the prior art motor-driven power steering system as previously referred to,
however, if there is failure in the vehicle speed sensor 10 or breaks in the wiring
connecting between the speed sensor 10 and the control unit 109, it becomes impossible
to control the power steering system in accordance with the travelling speed of the
vehicle so that the steering system is continuously held in a power steering mode
even when the vehicle travels at high speed. As a result, the steering operation for
the operator becomes too lighter, thus reducing safety in driving at high speed.
[0017] In this case, however, according to the present invention, the power steering system
is changed into a manual steering mode so as to avoid the above situation. To this
end, according to this embodiment, failure in the vehicle speed sensor 10 and/or breaks
in the wiring connecting the speed sensor 10 to the control unit 109 is first detected
in the following manner. Namely, as illustrated in Fig. 2, the output signal of the
vehicle height sensor 18 is input to the vehicle height measuring means 109c which
outputs to the vehicle travel judging means 109d a vehicle height signal having a
variation frequency f
H and a variation amplitude A
H which correspond to the measured vehicle height H. The vehicle travel judging means
109d judges from the vehicle height signal of the vehicle height measuring means 109c
whether or not the vehicle is travelling. If the variation frequency f
H and the variation amplitude A
H of the vehicle height signal continue to be greater than respective prescribed reference
values (for example, 0.5 Hz and ± I cm) for more than a prescribed time duration (for
example, 5 seconds), it is judged that the vehicle is travelling. In this state, if
the vehicle speed V as measured by the vehicle speed measuring means 109b is zero,
that is if the output signal of the vehicle speed measuring means 109b is zero, the
speed-sensor failure determining means 109e determines that there is failure in the
speed sensor 18 and/or breaks in the wiring connecting between the speed sensor 18
and the control unit 109, and operates, if so determined, to make the current supplied
to the motor 13 and the voltage imposed on the electromagnetic switching clutch 16
into zero, thereby changing the steering system into a non-power assisted manual steering
mode.
[0018] More specifically, as illustrated in the flow chart of Fig. 5, the control unit 109
is initialized after the key or ignition switch is turned on, and then, at step SI,
from the output signal of the steering torque sensor 3, the steering torque measuring
means 109a measures the steering torque imparted to the steering wheel I by the operator.
At step S2, from the output signal of the vehicle-speed sensor 10, the vehicle-speed
measuring means 109b measures the travelling speed V of the vehicle and generates
an output signal representative of the measured vehicle speed V, and at step S3, from
the output signal of the vehicle height sensor 18, the vehicle height measuring means
109c measures the height H of the vehicle from the ground and generates a vehicle
height signal having a variation frequency f
H and a variation amplitude As which correspond to the measured vehicle height. At
step S4, it is determined that whether or not the measured vehicle speed V is equal
to zero, and if not, the control process proceeds to step S5 where a motor current
corresponding to the measured steering torque Ts is read out. Then, at step S6, it
is determined whether or not the vehicle speed V is greater than a first reference
speed Vi, and if V > Vi, the control process proceeds to step S15, and if V ≦ Vi,
the control process proceeds to step S7 where it is further determined whether or
not the vehicle speed V is greater than a second reference speed V
2 which is smaller than the first reference speed Vi. If V > V
2, the motor current is reduced to an offset current levelloF (for example 2 -10 ampere),
and the clutch 16 is connected at step S9. At step S7, however, if V ≦ V
2, the control process proceeds to step SIO where the motor current 1
M corresponding to the measured steering torque Ts is output, and then the control
process proceeds to step S9.
[0019] On the other hand, at step S4, if it is judged that the vehicle speed is equal to
zero, the control process proceeds to step SII where the variation frequency f
H and the variation amplitude A
H of the vehicle height signal H is calculated. Then, at step SI2, it is determined
whether or not the calculated variation frequency f
H of the vehicle height signal H is greater than a prescribed reference frequency fs,
and if not, the control process proceeds to step S5 but if so, at step SI3, it is
determined whether or not the calculated variation amplitude A
HOf the vehicle height signal H is greater than a prescribed reference amplitude As.
If not, the control process proceeds to step S5, but if so, at step 14, it is determined
whether or not the time duration to for which the condition of f
H > fs and A
H > As continues is greater than a reference time ts. If t
D ≦ ts, the control process proceeds to step S5, but if to > ts, then the control process
proceeds to step SI5 where the current supplied to the motor 13 is made zero, and
at step SI6, the voltage imposed on the electromagnetic switching clutch 16 is also
made zero thereby to disconnect the clutch 16. After steps S9 and S16, the control
process returns to step SI, and the above-described operations are repeated.
[0020] Fig. 6 shows another embodiment of the present invention which has a different control
unit 109'. In this embodiment, in place of the vehicle height sensor 18 of the previous
embodiment, there is provided a throttle opening sensor 18' for sensing the opening
degree of an unillustrated throttle valve disposed in an unillustrated intake passage
of an engine to generate an output signal representative of the detected throttle
opening degree, and the control unit 109' includes, in addition to the same components
as those of the previous embodiment denoted by reference numerals 109a, 109b, and
109d through 109h, a throttle opening measuring means 109i in place of the vehicle
height measuring means 109c and the vehicle travel judging means 109d. The throttle
opening measuring means 109i is adapted to receive the output signal from the throttle
opening sensor 18' for measuring the opening degree So of the throttle valve. The
speed-sensor failure determining means 109e is adapted to receive the output signals
of the vehicle speed measuring means 109b and the throttle opening measuring means
109i and determine whether or not there is failure in the vehicle speed sensor 10
or breaks in the wiring connecting between the vehicle speed sensor 10 and the control
unit 109'. In this regard, if the measured opening degree So of the throttle valve
is greater than a reference throttle opening Ss (for example, 5 - 80% of the full
throttle opening) and if zero output of the vehicle speed measuring means 109b continues
for a time duration to', greater than a reference time ts', (for example, 10 - 20
seconds), the speed-sensor failure determining means 109e determines that there is
failure in the speed sensor 10 and/or breaks in the wiring connecting between the
speed sensor 10 and the control unit 109', and operates to make both the motor current
and the clutch voltage into zero, thereby changing the steering system into a non-power
assisted manual steering mode.
[0021] Fig. 7 is the flow chart showing the control process according to the embodiment
of Fig. 6. This flow chart is substantially similar to the flow chart of Fig. 5 except
for the following. Namely, in this flow chart, the step S3 of Fig. 5 is replaced by
step S3', and the steps SII through SI4 of Fig. 5 are replaced by steps SI7 and S18.
At step S3', the throttle opening measuring means 109i measures from the output signal
of the throttle opening sensor 18' the opening degree So of an unillustrated throttle
valve. At step 4, it is determined whether or not the measured vehicle speed V is
equal to zero, and if not, the control process proceeds to step S5. On the other hand,
if the vehicle speed V is zero, then the control process proceeds to step SI7 where
the measured opening degree So of the throttle valve is compared with a reference
throttle opening Ss. If So ≦ Ss, the control process proceeds to step S5, but if So
> Ss, then at step S18, the time duration t
D' for which zero output of the vehicle speed measuring means 109b continues is compared
with a reference time ts'. If t
D' ≦ ts', the control process proceeds to step S5, but if t
D' > ts', the control process proceeds to step S15. The control process of this flow
chart other than the above is identical to that of Fig. 5 and hence further detailed
description thereof will be unnecessary.
1. A power steering system for a vehicle comprising means (13,14,15,17), including
a motor, provided for transmitting a power steering force to the wheels of the vehicle
thereby to assist the steering motion of the vehicle, and control means (9) for controlling
the transmission of the power steering force in dependence upon the speed of the vehicle,
characterised in that the control means (9) is operative for disenabling the transmission
of the power steering force in the event that the control means (9) fails to receive
speed signals indicative of the speed of the vehicle.
2. A power steering system for a vehicle according to claim I, wherein said means
(13,14,15,17) includes a clutch (16) which is disengaged so as to prevent transmission
of the power steering force when the control means (9) fails to receive the speed
signals.
3. A power steering system for a vehicle according to claim 1 or claim 2, wherein
the power to the motor (13) is reduced to zero when the control means (9) fails to
receive the speed signals.
4. A power steering system for a vehicle according to claim I, claim 2 or claim 3,
wherein motion determining means (109) is provided for determining whether the vehicle
is in motion, wherein disenabling of the transmission of the power steering force
is conditional upon both failure of the control means (9) to receive the speed siqnals
and the determining means (109d) determining that the vehicle is in motion.
5. A power steering system for a vehicle according to claim 4, wherein the motion
determining means (109d) detects motion of the vehicle in dependence upon the degree
of opening of a throttle valve measured by a throttle opening measuring means (109i)
and/or changes in vehicle height measured by a vehicle height sensor (18,109c).
6. A motor-driven power steering system for a vehicle having a steering wheel operatively
connected with steerable road wheels so that the steering wheel is turned by an operator
to appropriately steer said steerable road wheels, said motor-driven power steering
system comprising:
a torque sensor for detecting steering force imparted to said steering wheel by an
operator to generate an output signal representative of the detected steering force;
a vehicle speed sensor for detecting the vehicle speed to generate an output signal
representative of the detected vehicle speed;
a motor adapted to be energized by a source of electric power for power assisting
the steering motion . caused by the operator through said steering wheel;
a clutch means adapted to be switched off for interrupting the transmission of the
power-assisting force from said motor toward said steerable road wheels when the vehicle
speed increases above a prescribed level, and switched on for establishing the transmission
of the power-steering force when the vehicle speed decreases below the prescribed
level; and
a control unit connected through wirings to said torque sensor and said vehicle-speed
sensor for controlling the operations of said motor and said clutch means based on
the output signals thereof in a manner such that the power-assisting force transmitted
from said motor to said steerable road wheels is controlled in accordance with the
travelling speed of the vehicle and the steering torque imparted to the steering wheel
by the operator under the normal operating condition of said vehicle speed sensor,
but reduced to zero if said vehicle speed sensor has failed and/or said wiring connecting
between said vehicle speed sensor and said control unit is broken.
7. A motor-driven power steering system for a vehicle as set forth in claim 6, wherein
the current supplied from said source of electric power to said motor is reduced to
zero by said control unit in case of failure in said vehicle speed sensor and/or breaks
in said wiring connecting between said vehicle speed sensor and said control unit.
8. A motor-driven power steering system for a vehicle as set forth in claim 6, wherein
said clutch means is switched off by said control unit in case of failure in said
vehicle speed sensor and/or breaks in said wiring connecting between said vehicle
speed sensor and said control unit.
9. A motor-driven power steering system for a vehicle as set forth in claim 6, further
comprising a vehicle height sensor adapted to detect the height of the vehicle from
the ground and generate an output signal representative of the detected vehicle height;
and wherein said control unit comprises:
a steering torque measuring means adapted to receive the output signal from said torque
sensor for measuring the operator-induced steering torque;
a vehicle speed measuring means adapted to receive the output signal from said vehicle-speed
sensor for measuring the vehicle speed;
a vehicle height measuring means adapted to receive the output signal from said vehicle
height sensor and generate an output signal having a variation frequency and a variation
amplitude which correspond to the measured vehicle height;
a vehicle travel judging means adapted to receive the output signal of said vehicle
height measuring means and judge that the vehicle is travelling when the variation
frequency and the variation amplitude of the output signal of said vehicle height
measuring means are continuously greater than respective prescribed reference values
for a time duration exceeding a prescribed period;
a speed-sensor failure determining means adapted to receive the output signals of
said vehicle speed measuring means and said vehicle travel judging means and determine
that there is failure in said vehicle speed sensor and/or breaks in said wiring connecting
between said vehicle speed sensor and said control unit when the output of said vehicle
speed measuring means is zero and when said vehicle travel judging means judges that
the vehicle is travelling;
a motor-current determining means adapted to determine an appropriate level of current
supplied to said motor in a manner such that the motor current is determined on the
basis of the output signals of said steering torque measuring means and said vehicle
speed measuring means in the normal operating condition of said vehicle speed sensor,
but made zero when said speed-sensor failure determining means determines that there
is failure in said vehicle speed sensor and/or breaks in said wiring connecting between
said vehicle speed sensor and said control unit;
a motor-current controlling means for controlling the current supplied to said motor
in accordance with the output from said motor-current determining means; and
a clutch controlling means for controlling the operation of said clutch means in a
manner such that said clutch means is switched off when the measured vehicle speed
is above the prescribed level or when said speed-sensor failure determining means
determines that there is failure in said vehicle speed sensor and/or breaks in said
wiring connecting between said vehicle speed sensor and said control unit, and switched
on when the measured vehicle speed is below the prescribed level.
10. A motor-driven power steering system for a vehicle as set forth in claim 6. further
comprising:
a throttle valve disposed in an intake passage of the engine for controlling the flow
rate of air sucked into the engine; and
a throttle opening sensor adapted to detect the opening degree of said throttle valve
and generate an output signal representative of the detected throttle opening degree;
and wherein said control unit comprises:
a steering torque measuring means adapted to receive the output signal from said torque
sensor for measuring the operator-induced steering torque;
a vehicle speed measuring means adapted to receive the output signal from said vehicle
speed sensor for measuring the vehicle speed;
a throttle opening measuring means adapted to receive the output signal from said
throttle opening sensor for measuring the opening degree of said throttle valve;
a speed-sensor failure determining means adapted to receive the output signals of
said vehicle speed measuring means and said throttle opening measuring means and determine
that there is failure in said vehicle speed sensor and/or breaks in said wiring connecting
between said vehicle speed sensor and said control unit when the opening degree of
said throttle valve measured by said throttle opening measuring means is greater than
a prescribed reference value and when zero output of said vehicle speed measuring
means continues for a time duration exceeding a prescribed period;
a motor-current determining means adapted to determine an appropriate level of current
supplied to said motor in a manner such that the motor current is determined on the
basis of the output signals of said steering torque measuring means and said vehicle
speed measuring means in the normal operating condition of said vehicle speed sensor,
but made zero when said speed-sensor failure determining means determines that there
is failure in said vehicle speed sensor and/or breaks in said wiring connecting between
said vehicle speed sensor and said control unit;
a motor-current controlling means for controlling the current supplied to said motor
in accordance with the output from said motor-current determining means; and
a clutch controlling means for controlling the operation of said clutch means in a
manner such that said clutch means is switched off when the measured vehicle speed
is above the prescribed level or when said speed-sensor failure determining means
determines that there is failure in said vehicle speed sensor and/or breaks in said
wiring connecting between said vehicle speed sensor and said control unit, and switched
on when the measured vehicle speed is below the prescribed level.